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AR24027
Operator’s Manual
2-2
2.1.3
Point to point topology
In a point-to-point topology you only have two radios establishing a single link. When the end points
are fixed we recommend using directional antennas at both ends, pointing at each other. This
increases the signal strength in the desired direction and shields the radios against unwanted
interference from other sources.
The point-to-point topology operates like a point-to-multipoint network where the hub has a single
remote. You still need to configure one of the two radios to be the hub. Hub radios can be
configured to accept a specific maximum number of remotes (see command “
node
”). By setting this
parameter to one the link is optimized for operation in point-to-point.
2.2
Time Division Duplex
2.2.1
Fixed and varibale cycle split
The AR24027 radio operates in Time Division Duplex (TDD) mode meaning that the radio operates
in a
cycle
consisting of two phases: the first phase is used for
outbound
transmissions (from hub to
remotes) and the second phase for
inbound
transmissions (from the remotes to the hub). Total cycle
time is fixed at 20 ms. However the AR24027 provides great flexibility in determining the “split”
between the outbound and inbound phases.
At the hub radio you can configure the cycle split in the following ways:
Fixed cycle split
: You can specify the cycle split in 10% nominal increments from 10/90
(outbound/inbound) all the way to 90/10. The advantage of a fixed TDD split is that it allows co-
locating multiple radios and completely avoiding self-generated interference. This is achieved
through the PULSAR Network Technology explained in section 2.3. The fixed split may also be
appropriate in applications where the data traffic is constant and with pre-determined throughput.
Automatic cycle split
: in this mode the hub radio changes the cycle split dynamically based on the
amount of traffic queued up in each direction. If you have a point-to-point link or a point-to-
multipoint network with no co-located radios and bursty traffic, then the automatic cycle split will
typically deliver the best performance.
2.2.2
On demand bandwidth allocation
The complete TDD cycle is divided into 20 slots of approximately just under 1 ms each. In automatic
cycle split mode, the hub examines the total traffic queued up for outbound and inbound, and selects
an appropriate cycle split. With fixed cycle split this step is omitted.
For the outbound traffic, the hub radio allocates the bandwidth on demand to each remote. If there is
no traffic to a specific remote, the hub does not transmit any packets to that remote. When the hub
has packets to multiple remotes, it distributes the available bandwidth evenly so that all remotes get
equal throughput.
The hub starts every outbound transmission with a broadcast packet that includes the current cycle
split as well as the slot allocation for the inbound phase. All remotes decode this packet and only
transmit if they have been assigned one or more slots during the inbound phase.
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